The present invention is directed to glass containers for food products and the like, and more particularly to a laminated glass container and method of manufacture in which the glass layers are of chemically incompatible composition, such as an oxidized flint glass outer layer over a reduced amber glass inner layer.
Glass containers are conventionally provided of either clear or colored construction. Clear containers of flint glass composition, for example, are employed for enclosing food products in applications in which it is desirable to be able to view the product through the container wall. On the other hand, amber glass is employed for products such as beer, for example, so that the absorption properties of the glass will decrease transmission of light wavelengths through the container that potentially might degrade the product.
It is also known that flint glass and amber glass are reactive with each other due at least in part to the differing oxidation states of these glasses. One accepted technique for quantifying these differing oxidation states involves use of a so-called xe2x80x9credox nurnberxe2x80x9d as described in Simpson and Myers, xe2x80x9cThe Redox Number Concept and Its Use by the Glass Technologist,xe2x80x9d Glass Technology, Vol. 19, No. 4., Aug. 4, 1978, pages 82-85. Amber glass may be characterized as a reduced glass, having a redox number in the negative range. On the other hand, flint glass may be characterized as an oxidized glass, having a redox number of zero or above. The chemistry for the amber glass coloration is complicated, and is believed to include a complex chromophore of ferric ion and sulfide ions, which is formed in a reducing atmosphere. At forming temperatures at which the glasses are molten, the flint and amber glasses will react with each other forming bubbles or foam in an oxidation/reduction reaction. It is thus necessary when converting a glass container manufacturing line from manufacture of flint glass containers to amber glass containers, for example, fully to remove any residual flint glass in the furnace and forehearth equipment to avoid formation of bubbles in the amber glass containers through redox reactions within the amber glass.
U.S. Pat. No. 4,740,401 discloses an apparatus and method for forming laminated glass containers that comprise an inner core glass surrounded by an outer skin or casing glass. The techniques disclosed in this patent have been employed for manufacture of flint glass containersxe2x80x94i.e., containers having a core layer of flint glass surrounded by a casing layer of flint glass having a lower coefficient of thermal expansion so that, on cooling, the casing layer is placed under compression and provides additional strength to the overall container sidewall. However, problems are to be anticipated when it is considered to employ this technology to form containers having an amber would glass core layer surrounded by an amber glass casing layer. The lower radiation heat transfer characteristics of amber glass make temperature control more difficult for the casing glass because of the relatively low flow rate of the casing glass. Furthermore, any residual flint glass or other contaminants in the casing glass forehearth or other flow equipment could cause production of seeds and blisters through redox reactions for reasons previously indicated. Moreover, the reduced glass composition of the amber glass may cause erosion of precious metal layers of platinum and rhodium in the glass melting system. There would also be substantial expense associated with more frequent replacement of casing glass flow equipment when used with amber glass rather than flint glass. It is therefore a general object of the present invention to provide a method of forming a laminated glass container that addresses one or more of these problems. Another and more specific object of the invention is to provide a method of constructing a laminated amber glass container. A further object of the present invention is to provide a glass container formed by such method.
Briefly stated, and by way of summary and not limitation, it has been found in accordance with the present invention that laminated glass containers of the character described above can be formed with an inner core layer of amber glass and an outer casing layer of flint glass employing manufacturing equipment and techniques of the type shown in U.S. Pat. No. 4,740,401 without producing blisters or bubbles at the interface between the amber core and flint casing layers. Specifically, it has been found that, employing such equipment and technology, the total time duration during which the flint and amber glasses are together at reactive (molten) temperatures is sufficiently small that the glass containers will be cooled below molten glass temperatures before the redox reactions have an opportunity to occur. Thus, laminated glass containers are provided having an inner core layer of amber glass and an outer casing layer of flint glass without production of the blisters or bubbles at the glass layer interface that would otherwise be expected to occur. Use of flint glass as the casing glass layer overcomes the problems that would otherwise be associated with use of amber casing glass as discussed abovexe2x80x94i.e., low radiation heat transfer characteristics, formation of seeds and blisters, and attack on precious metals employed in the glass melting system. The increased optical transmission of the overall container sidewall can be overcome, if needed, by decreasing the optical transmissibility of the amber glass inner layerxe2x80x94i.e., by increasing the intensity of the amber coloration.
There is thus provided in accordance with the present invention a method of forming a glass container that includes the steps of forming a cased glass stream that includes a core glass surrounded by a casing glass, with the core and casing glasses being of chemical compositions that react with each other at molten glass temperatures. Specifically, in the preferred embodiment of the invention, the glass stream includes a reduced core glass, preferably amber glass, and an oxidized casing glass, preferably flint glass. In the preferred implementation of the invention, the cased glass stream is formed into a glass container by cutting a mold charge gob from the stream and molding the gob into a laminated glass container sufficiently rapidly that the glasses do not have an opportunity to react with each other. When implementing the invention in conjunction with an individual section glassware forming machine, it is preferred to form multiple cased glass streams, multiple gobs and multiple glass containers simultaneously in each section of the machine.
In accordance with another aspect of the present invention, there is provided a laminated glass container that comprises an inner layer of amber glass surrounded by an outer layer of flint glass.